I. Field of the Invention
This invention relates generally to pacing and sensing leads for use with implantable cardiac stimulators, such as pacers and automatic implantable cardiac defibrillators (AICD), and more particularly to a bipolar screw-in type myocardial electrode system offering improved performance over prior art designs.
II. Discussion of the Prior Art
In the U.S. Pat. No. 4,010,758 to Ronald H. Rockland et al and entitled "Bipolar Body Tissue Electrode", there is described a myocardial screw-in bipolar tissue stimulating lead comprising a pair of elongated flexible conductors insulated from one another and extending from a proximal connector to an enlarged electrode supporting head at the distal end of the lead. Projecting outwardly from the head structure is a rigid helix formed from wire and which resembles a corkscrew. Surrounding the helical electrode is an apertured disk, resembling a common flat-washer, but of a suitable electrode material. The helix extends through the opening in the disk in non-contacting relationship. In use, it is intended that the Rockland electrode be screwed into epicardial tissue to the point where the flat disk electrode abuts the surface of the myocardium.
The lead described in the Rockland patent is intended strictly for stimulating cardiac tissue. That is to say, the proximal connector is intended to be coupled to a pulse generator, such as an implanted pacemaker which then functions to deliver a stimulating voltage between the corkscrew electrode and the surrounding disk electrode to the tissue located therebetween. No mention is made in the patent of using that arrangement as a sensing electrode whereby cardiac depolarization signals may be picked up and delivered to the implantable pacemaker for controlling the operation thereof.
In prior art, AICD systems of the type manufactured and sold by Cardiac Pacemakers, Inc. (applicant's assignee), it has been the practice to often utilize two monopolar positive fixation leads, e.g., corkscrew tips cooperating with myocardial tissue, for sensing ECG signals produced upon depolarization of that tissue and using the resulting signals, so detected, in determining whether normal sinus rhythm, tachycardia or ventricular fibrillation is in progress whereby the implanted pulse generator can be commanded to apply an appropriate shock or other stimulation to the heart over separate electrodes to restore sinus rhythm. It is also most important to proper operation of an AICD system that the sensing apparatus recover substantially instantaneously from a previous shocking episode whereby the pulse generator circuitry can determine whether defibrillation has, in fact, taken place or whether it will be necessary to recharge the device's capacitors in anticipation of the need for another shock to be administered. It is also known that antitachycardia pacing schemes may be employed to electrically treat tachycardia. Also, the heart may require short term back-up pacing following conversion attempts. In both cases, the electrode configuration used does not necessarily have to be designed or optimized for long term, chronic stimulation. In such cases, a lead designed for sensing may be suitable for delivery of these short term therapies.
In that the insertion of a positive fixation corkscrew electrode into the myocardial tissue is, at least, modestly traumatic, it is desirable, where possible, that only a single puncture be made. Thus, a bipolar electrode structure, rather than the use of two monopolar electrodes, is advantageous. To those familiar with the literature, it is known that a small surface area electrode favors pacing performance while a large surface area electrode favors sensing performance. These conflicting requirements, most recently pointed out by Sinnaeve, et. al., (PACE, volume 10, 1987, p. 546-554), makes the design of a single electrode, optimized for both purposes, especially difficult.